Project summary. A goal of our research is to understand mechanisms through which the immune system influences the pathology of Duchenne muscular dystrophy (DMD). As our understanding of the immunobiology of muscular dystrophy has advanced, clear evidence shows that the immune response to dystrophic muscle promotes muscle damage, at least in some stages of the disease. That knowledge has led to the expectation that suppressing the immune response can reduce the rate of progress or severity of the disease. However, a potential danger of using non-specific immunosuppressants to treat muscular dystrophy exists. The immune system can also promote tissue repair so that the broad application of immunosuppression as a treatment may have a net, negative effect on muscle health. Thus, a critical barrier to developing immune-based treatments for muscular dystrophy is the shortage of mechanistic knowledge of the diverse, complex and dynamic interactions between dystrophic muscle and the immune system. The focus of our study will be to determine the role of immune cells in the myeloid lineage for regulating muscle regeneration and test the novel hypothesis that myeloid cell production of the protein Klotho drives regeneration. We will pursue the following aims, using the mdx mouse model of DMD: Aim 1. Test the hypothesis that a protein expressed by M2 macrophages, called Klotho, promotes regeneration of dystrophic muscle. Aim 2. Test the hypothesis that Klotho modulates muscle inflammation in dystrophic mice. Aim 3. Test the hypothesis that increased expression of Klotho reduces pathology in severe muscular dystrophy caused by mutation of both dystrophin and utrophin. Collectively, these findings will provide new understandings concerning the role of macrophages in the regeneration of dystrophic muscle. We anticipate that this information can provide the foundation for future work in which the pro-regenerative influences of the immune system on dystrophic muscle can be exploited to reduce the pathology of DMD.